The present invention relates to a soldering apparatus of a reflow type and, more particularly, to a soldering apparatus of a reflow type in which electric parts such as chips temporarily mounted on a printed circuit board are soldered with solder preforms or solder pastes in a reflow chamber of the soldering apparatus after preheating them in a preheating chamber thereof.
A conventional soldering apparatus of the reflow type is arranged in which printed circuit boards on which chips have been temporarily mounted with solder preforms, solder pastes or adhesives are exposed to heated air or far infrared rays to fuse or melt the solder preforms or the like, thus soldering the chips on the printed circuit boards.
One conventional soldering apparatus of the reflow type is shown in FIG. 5 in which printed circuit boards 101 with chips 102 temporarily mounted thereon with solder pastes 103 are placed on a belt conveyor 105 of a metal mesh being conveyed on a direction shown by the arrow A. The soldering apparatus 104 is provided with a first preheating chamber 106 and a second preheating chamber 107 at positions along the direction of conveyance of the belt conveyor 105 and symmetrically above and below the belt conveyor 105. The first and second preheating chambers 106 and 107 are provided each with an air inlet 106a and 107a and a discharge outlet 106b and 107b, respectively. Air is sucked into the chambers 106 and 107 through the air inlets 106a and 107a, respectively, and the air is heated with a heater 109, such as an electric sheathed heater or a far infrared rays heater, mounted at the respective discharge outlets 106b and 107b, to temperatures as high as approximately 140.degree. C., for example, below the fusion point of the solder pastes 103 or the like, thus preheating the printed circuit boards 101 being conveyed in the chambers 106 and 107. The preheated printed circuit boards 101 are then conveyed to a reflow chamber 108 in the soldering apparatus 104, which is provided at a discharge exit 108b thereof with a heater 110, such as an electric sheathed heater or far infrared rays heater. The printed circuit boards 101 are in turn heated with air sucked from an air inlet 108a and heated by the heater 110 at temperatures as high as 215.degree. C., at the fusion point of the solder pastes or the like or higher, thus soldering the chips 102 on the printed circuit boards 101. The heated air is discharged through the discharge outlet 108b. The preheating chambers 106 and 107 and the reflow chamber 108 are further provided each with a fan 111 which is connected through a rotary shaft 112 to each other and to a motor 113. The fans 111 then ventillate heated air in and from the chambers in a direction shown by the arrow B. The soldering apparatus 104 is also provided with a cooling fan 114 for cooling the fused or melted solder pastes 103 to solidify them and ensuring the fixed mounting of the chips 102 to the printed circuit boards 101.
The heater 109 or 110, such as an electric sheathed heater or a far infrared rays heater, generates radiant heat containing far infrared rays, and the far infrared rays have the advantage that they are likely to heat even the inside of the chips 102 as compared with a heating system applying a thermal conduction of heated air. It is to be noted, however, that, as the solder pastes 103 have a larger reflectance than the chips 102, the far infrared rays require a longer time for heating the solder pastes 103 than for heating the chips 102, whereby the chips 102 are heated earlier than the solder pastes 103 and it takes a longer time to solder the chips 102.
The heating system using the heater 109 or 110 is more difficult in setting heating requirements than the heating system using heated air because there are many varieties in kinds of printed circuit boards 101 and their sizes.
Furthermore, when a reflecting plate is provided for reflecting heat from the heater 109, 110, the reflecting surface may become coated with flux scattered from the solder pastes 103, reducing the thermal efficiency of the heater 109, 110.
The air heated by the heaters 109 in the first and second preheating chambers 106 and 107 is ventillated downwardly by the ventillating fan 111, but this causes a turbulence to occur. The heating by the air in a turbulent state in association with the heater 109, 110 has the tendency to cause irregularity in heating of the printed circuit boards 101, particularly in the direction perpendicular to a direction (shown by the arrow A) of conveyance of the printed circuit boards 101, namely, in a widthwise direction thereof. In other words, a difference in temperatures may be likely to occur between a middle portion of the printed circuit board 101 and both end portions in a widthwise direction thereof. If such printed circuit boards 101 are conveyed for soldering to the reflow chamber 108 where the air is likewise in a turbulent flow, the solder pastes 103 are fused in such an irregular state that uniform soldering cannot be ensured.
Accordingly, it is required to block impact upon printed circuit boards 101 of radiant heat including far infrared rays generated from the heater 109, 110.
In order to ensure good adherence of the solder pastes 103 to the printed circuit boards 101, the solder pastes 103 should be heated rapidly and thereafter cooled rapidly. It is to be noted, however, that, when printed circuit boards 101 and chips 102 at ambient temperatures as high as approxmately 20.degree. C. are rapidly heated by direct contact with air heated to temperatures as high as 215.degree. C. or exposure to far infrared rays, they are likely to undergo heat shock causing damages. Further, fluxes and volatile ingredients in the solder pastes 103 are caused to evaporate upon fusion by heating and air bubbles may be formed within the solder pastes 103. However, a portion of such air bubbles remains trapped within the solder pastes 103 rendering the pastes porous, thus decreasing the bonding intensity of the solder pastes 103. Furthermore, air bubbles generated at the surface portions of solder pastes 103 scatter fused solder pastes 103 which adhere to wiring of chips 102 and print circuit boards 101, thereby damaging the chips 102 and short-circuiting the wiring. These disadvantages impair the reliability and quality of printed circuit boards 101.
For the conventional soldering apparatus 104, the air heated in the preheating chambers 106, 107 and the reflow chamber 108 and withdrawn once therefrom is then directed back to the ventillating fans 111 through flow passages disposed in the chambers 106, 107, 108 after the air has heated the solder pastes 103 in the chambers 106, 107, 108. It is to be noted that fusion of the solder pastes 103 with the heated air generates smoke or fumes and odorous gases upon heating of the fluxes or the like. The smoke and gases are directed by the ventillating fans 111 to the preheating chambers 106, 107 and the reflow chamber 108 together with the heated air through the flow passages. As a matter of course, these smoke and gases cause dirt to adhere on the inner wall surfaces of the chambers 106, 107, 108, the heaters 109, 110, the fans 111 and the withdrawing duct 115. Furthermore, a portion of the smoke and gases escapes outside the soldering apparatus 104 of the reflow type, impairing operational circumstances, causing pollution and so on.
It is thus required to remove the smoke or fumes and the gases.